In today's high speed electronic equipment, it is desirable that all components in an interconnection path be optimized for signal transmission characteristics, otherwise the performance of the system will be impaired or degraded. Such characteristics include risetime degradation or system bandwidth, crosstalk, impedance control and propagation delay. Ideally, the characteristics of an electrical connector would have little or no affect on the interconnection system. An ideal connector would be "transparent". In other words, the system would function as if the connector did not exist as part of the interconnection. However, such an ideal connector is generally impractical or impossible, and continuous efforts are made to develop electrical connectors which have as little affect on the system as possible.
It has been found that inductance is one of the major concerns in designing an ideal connector. This is particularly true in electrical connectors for high speed electronic equipment, i.e. involving the transmission of high frequencies. A very popular type of electrical connector for such applications commonly is termed an "edge card" connector. In other words, an edge connector is provided for receiving a printed circuit board having a mating edge and a plurality of contact pads adjacent the edge. Such edge connectors have an elongated housing defining an elongated slot for receiving the mating edge of the printed circuit board. A plurality of terminals are spaced along one or both sides of the slot for engaging the contact pads adjacent the mating edge of the board. Most often, the terminals have some form or another of spring contact elements for biased engagement against the contact pads on the board and, most often, the spring contact elements are in one form or another of a cantilevered spring arm.
With electrical connectors of the character described above, given electrical contacts of otherwise comparable geometry, the longer the spring contact arm or contact beam, the greater the inductance of the terminal and, cumulatively, the electrical connector itself. Therefore, it is desirable to have as short a contact beam as possible. Shortening the contact beam creates various problems. For instance, it is difficult to maintain constant contact pressures in a multiplicity of spring contact beams, particularly when the beams are relatively short. In addition, it is difficult to compensate for variances in the widths of printed circuit boards when the contact beams are short. In fact, such spring contacts can take a permanent set even when the contacts are displaced only a small amount. The contacts might take a permanent set after a relatively wide printed circuit board has been inserted into the connector. This permanent set of the contacts would make the connector ineffective when a relatively narrow board subsequently is inserted into the connector. If the contact beams do not make effective electrical connection with the contact pads on the edge of the printed circuit board, an unreliable or ineffective electrical connection results, rendering the connector effectively useless.
This invention is directed to solving these problems by providing an edge card connector of the character described wherein the deflection of the spring contact elements is controlled, resulting in the ability to design very short spring contact beams.